Thanks for the info . Its known to me that horn flare provides some acoustic resistance to the compression drivers my question /worry is asy this resistance isnt present then will the diaphragm will be understress? . Like for example if we run a motor pump (water) dry run its prone to damage . May be I am asking a funny question .

It won't be hurt if you don't run it beyond its limits, but its limits have drastically changed. It is missing the acoustic loading it is designed for and will not produce anywhere near the efficiency, which may cause you to turn it up and blow it. It is also designed expecting the front air load to damp its excursion a bit, so you have danger from that, too.
Dan

Pretty often the performance of a compression driver is offered with it mated to a particular horn. A different horn or no horn is going to change everything. Particularly the diaphram excursion at the bottom of the band will increase a lot with no horn. If you can hear very well at all the driver will tell you when you are running out of linear excursion. If you can't hear very well and operate long with too much excursion, the driver will eventually break the suspension, making noises that pretty clearly indicate something has gone wrong. (or break the coil leads and stop making any noise at all) If you want to investigate what can be had from doing this try getting yourself a cheap driver and abuse it a little just for fun.